1. Field of the Invention
The present invention relates to a method for transmitting data in a closed-loop multiple input multiple output system, and more specifically, a method for transmitting information regarding transmission format in a closed-loop multiple input multiple output system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a popular wireless communication technology to multiplex data in frequency domain.
A multiple antenna communication system, which is often referred to as multiple input multiple output (MIMO) system, is widely used in combination with OFDM technology, in a wireless communication system to improve system performance.
In a MIMO system, both transmitter and receiver are equipped with multiple antennas. Therefore, the transmitter is capable of transmitting independent data streams simultaneously in the same frequency band. Unlike traditional means of increasing throughput (i.e., the amount of data transmitted per time unit) by increasing bandwidth or increasing overall transmit power, MIMO technology increases the spectral efficiency of a wireless communication system by exploiting the additional dimension of freedom in the space domain due to multiple antennas. Therefore MIMO technology can significantly increase the throughput and range of the system.
When the transmission channels between the transmitters and the receivers are relatively constant, it is possible to use a closed-loop MIMO scheme to further improve system performance. In a closed-loop MIMO system, the receiver informs the transmitter of feedback information regarding the channel condition. The transmitter utilizes this feedback information, together with other considerations such as scheduling priority, data and resource availability, to optimize the transmission scheme.
A popular closed-loop MIMO scheme is MIMO preceding. With precoding, the data streams to be transmitted are precoded, i.e., pre-multiplied by a precoding matrix, before being passed on to the multiple transmit antennas in a transmitter.
In a contemporary closed-loop MIMO preceding scheme, when a transmitter precodes data before transmitting the data to a receiver, the transmitter informs the receiver of the preceding information such as an identification of the preceding matrix by transmitting dedicated pilots (also referred to as reference signals) or explicit control information that carries the preceding information. A significant problem with this approach is that the control information inefficiently consumes a significant amount of system resources and degrades the overall system throughput and capacity.